Diarylide yellow pigments are well known disazo organic pigments. They are manufactured by tetrazotizing a benzidine to form the tetrazo salt, which is coupled with an acetoacetanilide coupling agent. Examples of acetoacetanilide coupling agents include acetoacetanilide (AAA), acetoacet-m-xylidide (AAMX), acetoacet-o-toluidide (AAOT), acetoacet-p-touidide (AAPT), acetoacet-o-anisidide (AAOA), acetoacet-o-chloroanilide (AAOCA), and acetoacet-2,5-dimethoxy-4-chloroanilide (AADMCA). Common benzidines are 3,3′-dichlorobenzidine (DCB), 2,2′,5,5′-tetrachlorobenzidine, 3,3′-dimethoxybenzidine, and 3,3′-dimethylbenzidine, but others are known.
Diarylide yellow pigments produced from DCB and acetoacetanilides constitute a family of pigments produced in large volumes for all the most common uses of organic pigments. Color Index Pigment Yellow 13 (C.I. Pigment Yellow 13), which is produced from tetrazotized 3,3′-dichlorobenzidine and two equivalents of AAMX, and Color Index Pigment Yellow 83 (C.I. Pigment Yellow 83), which is produced from tetrazotized 3,3′-dichlorobenzidine and two equivalents of AADMCA, are examples of such diarylide yellow pigments which possess somewhat different characteristics that will ultimately come from substituents on the acetoacetanilides.
When a mixture of acetoacetanilides is present at the reaction stage, the result will be a mix of pigments, many of which do not have Color Index status. U.S. Pat. No. 3,529,984 describes the formation of a mixture of three pigments when a mixture of two different acetoacetanilides is used, and U.S. Pat. No. 3,533,818 generalizes this concept to as many as six different acetoacetanilides. Both disclosures operate with minimum molar fractions of coupling agent down to approximately 5%. Pigments produced by these methods will exhibit improved application properties in printing inks and lacquers (e.g. increased tinctorial strength, transparency, gloss and lower rheology).
During the formation of C.I. Pigment Yellow 83, AADMCA decomposes resulting in some aromatic amine impurities. These impurities cause health concerns and regulatory compliance issues for products where Pigment Yellow 83 is incorporated. The acetoacetanilide coupling agent AAMX used to form C.I. Pigment Yellow 13 does not have the same stability problem as AADMCA, however, C.I. Pigment Yellow 13 has a much greener shade.
Lower levels of primary aromatic amines can be achieved with C.I. Pigment Yellow 83 by extensive purification but the byproduct amines must be treated and the purification process decreases product yield.
Consequently, a significant need exists for a pigment with a shade similar to C.I. Pigment Yellow 83, yet containing a low level of aromatic amine. There is also a need for a pigment that provides a combination of shade and durability.
An improved pigment may be produce by reacting a two equivalent mixture of AAMX and AADMCA with tetrazotized DCB. The pigment shows a surprisingly red shade over a large composition range, and exhibits an unexpected morphology.
These and other objects and advantages shall be made apparent from the accompanying figures and the description thereof.